The invention relates generally to a circuit for converting a signal from a capacitive sensor into a form suitable for signal processing. More specifically, the invention is a circuit for converting the output of a capacitive sensor into a sampled data output signal.
Capacitive sensors are used in applications such as accelerometers and pressure sensors. One type of capacitive sensor is manufactured by micromachining a slab of silicon to form a miniature tilt plate suspended above a substrate by torsion arms. The tilt plate has a center of mass offset from the torsion arm axis such that, under conditions of non-zero acceleration perpendicular to the plate, the plate tilts relative to the underlying substrate. The tilt plate is metallized and forms a common electrode for two capacitors. Two metallized regions are formed on the substrate directly beneath the suspended tilt plate and form the other electrodes of two capacitors with the tilt plate electrode. The geometry of the two metallized regions and the tilt plate is such that under acceleration the capacitance of one capacitor increases while the capacitance of the other decreases since the distance between the tilt plate and the metallized regions on the substrate either decreases or increases.
In operation, a capacitive tilt plate sensor connects one capacitor electrode from each capacitor at a common node. The common node provides an output of the sensor and the two remaining electrodes provide inputs to the sensor. If one capacitance value is denoted by C.sub.A and the other capacitance is value C.sub.B, then the output of the sensor is given by the formula: ##EQU1##
While the capacitances of C.sub.A and C.sub.B vary somewhat non-linearly with respect to acceleration, the above formula of the difference over the sum of the capacitors is remarkably linear with respect to acceleration. It is desirable therefore to have a circuit which produces an electrical output proportional to the quantity ##EQU2##
It is also desirable however to have the flexibility to scale the output voltage so that the circuit can be used in many applications. That is, if the output is scaled, the scale can be adjusted to be within the input range of downstream processing components without further processing.